CHAP. IL] THE CONTRACTILE TISSUES. 125 



latent-period changes, also form a series the members of which are 

 distinct. It is not until these molecular changes become trans- 

 formed into visible changes of form that any fusion or summation 

 takes place. 



73. Putting together the facts contained in this and the pre- 

 ceding sections, the following may be taken as a brief approximate 

 history of what takes place in a muscle and nerve when the latter 

 is subjected to a single induction-shock. At the instant that the 

 induced current passes into the nerve, changes occur, of whose 

 nature we know nothing certain except that they cause a ' current 

 of action ' or ' negative variation ' of the ' natural ' nerve-current. 

 These changes propagate themselves along the nerve in both 

 directions as a nervous impulse in the form of a wave, having 

 a wave-length of about 18 mm., and a velocity (in frog's nerve) of 

 about 28 m. per sec. Passing down the nerve fibres to the muscle, 

 flowing along the branching and narrowing tracts, the wave at last 

 breaks on the end-plates of the fibres of the muscle. Here it is 

 transmuted into what we may call a muscle impulse, with a shorter 

 steeper wave, and a greatly diminished velocity (about 3 m. per 

 sec.). This muscle impulse, of which we know hardly more than 

 that it is marked by a current of action, travels from each end- 

 plate in both directions to the end of the fibre, where it appears to 

 be lost, at all events we do not know what becomes of it. As this 

 impulse wave, whose development takes place entirely within the 

 latent period, leaves the end-plate it is followed by an explosive 

 decomposition of material, leading to a discharge of carbonic acid, 

 to the appearance of some substance or substances with an acid 

 reaction, and probably of other unknown things, with a consider- 

 able development of heat. This explosive decomposition gives rise 

 to the visible contraction wave, which travels behind the invisible 

 muscle impulse at about the same rate, but with a vastly increased 

 wave-length. The fibre, as the wave passes over it, swells and 

 shortens and thus brings its two ends nearer together. 



When repeated shocks are given, wave follows wave of nervous 

 impulse, muscle impulse, and visible contraction ; but the last do 

 not keep distinct, they are fused into the continued shortening 

 which we call tetanus. 



